Spherical Coated Magnesium Oxide Powder, Method for Producing the Same and Resin Composition Comprising the Powder

ABSTRACT

Provided is a sphere-shaped coated magnesium oxide powder having a surface coated with a double oxide and having an average shape factor of 1.25 or less, wherein the powder is advantageous not only in that it has excellent humidity resistance, but also in that it has excellent filling property and excellent flowability when being used as filler for a resin. In addition, provided is a method for producing a sphere-shaped coated magnesium oxide powder, wherein the method comprises allowing a compound of an element forming a double oxide to be present on the surface of magnesium oxide powder, and then fusing the resultant magnesium oxide powder at a high temperature so that the surface of the magnesium oxide powder is coated with the double oxide and the magnesium oxide powder is shaped into sphere. Further, provided are a resin composition comprising the sphere-shaped coated magnesium oxide powder and an electronic device using the resin composition.

FIELD OF THE INVENTION

The present invention relates to a sphere-shaped coated magnesium oxidepowder having excellent humidity resistance and excellent fillingproperty when being used as filler, and a method for producing the same.The present invention is also concerned with a resin compositioncomprising the sphere-shaped coated magnesium oxide powder and anelectronic device using the resin composition.

BACKGROUND OF THE INVENTION

An electronic device is consisted of electronic components, such as alaminate, a printed wiring board, and a multilayer wiring board. Inelectronic components, generally, a resin composition is used in aprepreg, a spacer, a sealer, or an adhesive sheet, and the resincomposition is required to have various performance and properties. Forexample, recently, an electronic device tends to have a large-capacitypower element mounted or have elements mounted with high density, andtherefore, a resin composition used in the electronic device and amaterial formed from the resin composition are required to have moreexcellent heat radiating property and humidity resistance than those ofconventional ones.

As filler for use in a semiconductor sealing resin composition, silicondioxide (hereinafter, referred to as “silica”) and aluminum oxide(hereinafter, referred to as “alumina”) have conventionally been used.However, silica has low thermal conductivity, and hence cannotsatisfactorily remove from the semiconductor a large amount of heatgenerated due to the increase of the integration degree, power, or speedof the device, thus causing a problem of the stable operation of thesemiconductor. On the other hand, when using alumina having thermalconductivity higher than that of silica, the heat radiating property isimproved; however, alumina, which has high hardness, poses a problem inthat it considerably wears a kneader, a molding machine, or a mold.

For solving the problems, with respect to the material for the fillerfor use in semiconductor sealing resin, studies are made on magnesiumoxide having thermal conductivity which is higher than that of silica byone order of magnitude and substantially equivalent to that of alumina.However, the magnesium oxide powder has high moisture absorption, ascompared to the silica powder. Therefore, when the magnesium oxidepowder is used as filler for semiconductor sealing resin, hydrationoccurs between the water absorbed and magnesium oxide, leading toproblems in that the filler increases in volume to cause cracks, andthat the thermal conductivity is lowered. For this reason, for surelyachieving a stable operation of the semiconductor for a long term, it isimportant to impart a humidity resistance to the magnesium oxide powderused as filler for semiconductor sealing resin.

As a method for obtaining magnesium oxide powder having an improvedhumidity resistance, Japanese Unexamined Patent Publication Nos.2003-34522 and 2003-34523 disclose a method for producing coatedmagnesium oxide powder, wherein the method comprises mixing an aluminumsalt or a silicon compound with magnesium oxide powder, and collectingsolids by filtration and drying and calcining the solids so that thesurface of the magnesium oxide powder is coated with a coating layercomprising a double oxide of aluminum or silicon and magnesium.

The coated magnesium oxide powder obtained by the above method isimproved in humidity resistance, but the particles of the powder haveangular shapes and hence have low filling property for a resin, andfurther there is a problem in that the resultant resin composition haslow flowability.

On the other hand, Japanese Patent No. 2590491 discloses a method forproducing a magnesium oxide material, in which alumina and/or silicaparticles are added to magnesium oxide powder, and the resultant mixtureis granulated using a spray dryer to obtain a sphere-shaped granulatedmaterial, and then at least part of the granulated material is fusedwithout destroying the granulated state, followed by quenching.

This method is intended to improve the magnesium oxide powder inhumidity resistance, but the sphere-shaped granulated material obtainedby granulation using a spray dryer is an aggregate of particles, thatis, a porous material, and therefore it is expected that the granulatedmaterial is difficult to fill a resin with high density.

It is an object of the present invention to solve the above problems andto provide a sphere-shaped coated magnesium oxide powder which isadvantageous not only in that it has excellent humidity resistance andexcellent filling property when being used as filler for a resin, butalso in that the resin composition containing the powder has highflowability and hence is excellent in moldability. It is another objectof the present invention to provide a method by which the sphere-shapedcoated magnesium oxide powder can be easily produced, and a resincomposition comprising the sphere-shaped coated magnesium oxide powderand an electronic device using the resin composition.

DISCLOSURE OF THE INVENTION

For attaining the above objects, the present inventors have madeextensive and intensive studies on the shape of a coated magnesium oxidepowder in respect of an average shape factor of powder particles. As aresult, it has been found that the average shape factor has an optimalrange, and thus the present invention has been completed.

Further, it has been found that a sphere-shaped coated magnesium oxidepowder can be easily produced by a method in which a compound of anelement forming a double oxide having a melting point of 2,773 K orlower is allowed to be present on the surface of magnesium oxide powderand the resultant magnesium oxide powder is fused at a high temperature.

Specifically, in the present invention, there is provided asphere-shaped coated magnesium oxide powder having a surface coated witha double oxide and having an average shape factor of 1.25 or less.

In addition, in the present invention, there is provided a method forproducing a sphere-shaped coated magnesium oxide powder, wherein themethod comprises allowing a compound of an element forming a doubleoxide to be present on the surface of magnesium oxide powder, and thenfusing the resultant magnesium oxide powder at a high temperature sothat the surface of the magnesium oxide powder is coated with the doubleoxide and the magnesium oxide powder is shaped into sphere.

Further, in the present invention, there are provided a resincomposition comprising the above sphere-shaped coated magnesium oxidepowder and an electronic device using the resin composition.

BEST MODE FOR CARRYING OUT THE INVENTION

Sphere-Shaped Coated Magnesium Oxide Powder

The sphere-shaped coated magnesium oxide powder of the present inventionhas a surface coated with a double oxide and has an average shape factorof 1.25 or less.

The shape factor is a value determined with respect to a projected imageof particles and represented by the formula: (Circumferencelength)²/{4π×(Sum of areas)}. The shape factor indicates that the valueis 1 when the projected image of particles is precisely a circle, andthat the value is larger when the projected image has an irregular form.

The average shape factor employed in the present invention is an averagevalue of the shape factors of 100 particles determined using a lasermicroscope and an image analysis software.

For increasing the rate of the powder filling a resin, it is requiredthat the shape of the powder be as spherical as possible. In the presentinvention, it is necessary that the sphere-shaped coated magnesium oxidepowder have an average shape factor of 1.25 or less, and the powderpreferably has an average shape factor of 1.22 or less, more preferably1.20 or less. The powder having an average shape factor of 1.25 or lessis improved in the filling property for a resin, and further theresultant resin composition has excellent flowability.

The sphere-shaped coated magnesium oxide powder of the present inventionhas a surface coated with a double oxide. The first purpose of thecoating with a double oxide is to improve the humidity resistance of themagnesium oxide powder, and the second purpose is to facilitate the stepfor causing the magnesium oxide powder to be shaped into sphere.Specifically, a double oxide having a melting point lower than the flametemperature is formed on the surface of the magnesium oxide powder tolower the melting point of the surface of the magnesium oxide powder,thus making it easy to cause the powder to be shaped into sphere. Themelting point of the double oxide is preferably 2,773 K or lower, morepreferably 2,273 K or lower.

It is preferred that the double oxide coating the surface of themagnesium oxide powder comprises at least one element selected from thegroup consisting of aluminum, iron, silicon, and titanium, andmagnesium. Examples of double oxides include forsterite (Mg₂SiO₄),spinel (Al₂MgO₄), magnesium ferrite (Fe₂MgO₄), and magnesium titanate(MgTiO₃).

The amount of the double oxide used in the present invention, that is,the amount of the double oxide present on the surface per one particleis preferably 5 to 50 mass %, more preferably 10 to 40 mass %. When theamount of the double oxide falls in the above range, the surface of themagnesium oxide powder is completely coated with the double oxide andhence the powder is easily shaped into sphere, so that the resultantpowder can fill a resin with high density, and further the resincomposition containing the powder has such high thermal conductivitythat it can exhibit satisfactory effects of thermal conductive filler.

The sphere-shaped coated magnesium oxide powder of the present inventionpreferably has an average particle size of 5×10⁻⁶ to 500×10⁻⁶ m, morepreferably 10×10⁻⁶ to 100×10⁻⁶ m. The sphere-shaped coated magnesiumoxide powder preferably has a BET specific surface area of 5.0×10³ m²/kgor less, more preferably 1×10³ m²/kg or less.

Method for Producing a Sphere-Shaped Coated Magnesium Oxide Powder

The sphere-shaped coated magnesium oxide powder of the present inventionis produced by allowing a compound of an element forming a double oxideto be present on the surface of magnesium oxide powder, and then fusingthe resultant magnesium oxide powder at a high temperature so that thesurface of the magnesium oxide powder is coated with the double oxideand the magnesium oxide powder is shaped into sphere.

Generally, as a method for obtaining powder having a particle shape asspherical as possible, for example, a method is used in which powder ispassed through a flame at a high temperature and fused and shaped intosphere due to a surface tension. This method can be applied to silica oralumina having a melting point lower than the flame temperature (2,073to 2,723 K) caused by burning of oxygen, but the method cannot beapplied to magnesium oxide having a melting point higher than the flametemperature (melting point: 3,073 K), and therefore it has beenconsidered that this method is difficult to cause the magnesium oxide tobe shaped into sphere.

In the method of the present invention, a double oxide having a meltingpoint lower than the flame temperature is formed on the surface ofmagnesium oxide powder to lower the melting point of the surface of themagnesium oxide powder, and therefore the above-mentionedhigh-temperature flame fusion process can be applied to cause themagnesium oxide powder to be shaped into sphere. The double oxidepreferably has a melting point of 2,773 K or lower, more preferably2,273 K or lower.

It is preferred that the compound used for forming a double oxide is atleast one compound selected from the group consisting of an aluminumcompound, an iron compound, a silicon compound, and a titanium compound.The form of the compound is not limited, but a nitrate, a sulfate, achloride, an oxynitrate, an oxysulfate, an oxychloride, a hydroxide, oran oxide is used.

It is preferred that the amount of the compound incorporated into themagnesium oxide powder is determined so that the content of the doubleoxide in the sphere-shaped coated magnesium oxide powder finallyobtained becomes 5 to 50 mass %.

When, for example, a silicon compound is used as the compound of anelement forming a double oxide, the sphere-shaped coated magnesium oxidepowder can be produced by a method in which fumed silica and magnesiumoxide powder are wet-blended together, and filtered and then dried toobtain magnesium oxide powder on which surface silica is uniformlyadsorbed, and the powder obtained is passed through a flame of, e.g.,propane-oxygen using oxygen as carrier gas, and cooled and thencollected by means of a collector. The combustible gas is not limited,but combustible gas, such as propane, butane, acetylene, or hydrogen, ormixed gas thereof can be used.

The magnesium oxide powder used in the present invention preferably hasa crystallite size of 50×10⁻⁹ m or more. The magnesium oxide powderhaving a crystallite size of 50×10⁻⁹ m or more has low reactivity, ascompared to more finely divided powder, and hence, e.g., a siliconcompound can be uniformly adsorbed on the surface of the magnesium oxidepowder, thus causing the particles of powder to be uniformly shaped intosphere. Further, the surface of the magnesium oxide powder can beuniformly coated with a double oxide, improving the powder in humidityresistance.

The crystallite size employed in the present invention is a valuedetermined from a Scherrer equation using X-ray diffractometry.Generally, an individual particle is a polycrystal comprised of aplurality of single crystals, and a crystallite size means an average ofthe sizes of single crystals constituting the polycrystal.

With respect to the purity of the magnesium oxide powder, there is noparticular limitation, and it is preferred to determine the puritydepending on the use of the powder. For example, for meeting theinsulating properties of electronic components, the magnesium oxidepowder preferably has a purity of 90% or more, more preferably a purityof 95% or more. The magnesium oxide powder having the properties in thepresent invention can be produced using a known process to a personskilled in the art, e.g., a fusion process or a sintering process.

By the above-described production method, a sphere-shaped coatedmagnesium oxide powder having high filling property for a resin can beeasily obtained at low cost while maintaining excellent humidityresistance and excellent thermal conductivity. In addition, a resincomposition comprising the thus obtained sphere-shaped coated magnesiumoxide powder has excellent flowability and improved moldability.

In the method of the present invention, the step of fusing and causingthe magnesium oxide powder to be shaped into sphere is not limited tothe above flame fusion step, and a step that can achieve a desiredtemperature, for example, a plasma heating step can be used.

Resin Composition Comprising the Coated Magnesium Oxide Powder

The resin composition of the present invention is obtained by adding theabove-described sphere-shaped coated magnesium oxide powder to a resin.

In this case, if necessary, the sphere-shaped coated magnesium oxidepowder of the present invention can be subjected to surface treatmentwith a silane coupling agent, a titanate coupling agent, or an aluminatecoupling agent, making it possible to further improve the fillingproperty.

Examples of silane coupling agents include vinyltrichlorosilane,vinyltrialkoxysilane, glycidoxypropyltrialkoxysilane, andmethacroxypropylmethyldialkoxysilane.

Examples of titanate coupling agents include isopropyltriisostearoyltitanate, tetraoctyl bis(ditridecylphosphite)titanate, andbis(dioctyl pyrophosphate)oxyacetate titanate.

With respect to the resin used in the resin composition of the presentinvention, there is no particular limitation, and examples includethermosetting resins, such as epoxy resins, phenolic resins, polyimideresins, polyester resins, and silicone resins, and thermoplastic resins,such as polycarbonate resins, acrylic resins, polyphenylene sulfideresins, and fluororesins. Of these, preferred are epoxy resins, siliconeresins, and polyphenylene sulfide resins. If necessary, a curing agentor a curing accelerator can be added.

Examples of epoxy resins include bisphenol A epoxy resins, novolak epoxyresins, bisphenol F epoxy resins, brominated epoxy resins, ortho-cresolnovolak epoxy resins, glycidyl ester resins, glycidyl amine resins, andheterocyclic epoxy resins.

Examples of phenolic resins include novolak phenolic resins and resolphenolic resins.

Examples of silicone resins include millable silicone rubbers, condensedliquid silicone rubbers, addition liquid silicone rubbers, and UV curingsilicone rubbers, and preferred are addition liquid silicone rubbers.Any of one-pack silicone rubbers and two-pack silicone rubbers may beused, but preferred are two-pack silicone rubbers.

In the resin composition of the present invention, in addition to thesphere-shaped coated magnesium oxide powder, other filler can be added.With respect to the other filler, there is no particular limitation, andexamples include fused silica and crystalline silica. If necessary, alubricant, a flame retardant, a coloring agent, or a low-stressimparting agent can be appropriately added.

The electronic device of the present invention uses the above-describedresin composition in its part, and has excellent heat radiating propertyand excellent humidity resistance. Examples of electronic devicesinclude a resin circuit board, a metal base circuit board, a metal-cladlaminate, and a metal-clad laminate having an inner circuit.

Examples of uses of the resin composition of the present invention inthe above electronic devices include a semiconductor sealer, a bondingagent, an adhesive sheet, a radiating sheet, a radiating spacer, and aradiating grease.

In the production of the above board using the resin composition of thepresent invention, a paper substrate or a glass substrate is immersed inthe resin composition of the present invention, and dried by heating tocure the resin into a B-stage, forming a prepreg (such as resin cloth orresin paper).

Further, using the prepreg, a resin circuit board, a metal-cladlaminate, or a metal-clad laminate having an inner circuit can beproduced. For example, a metal-clad laminate is produced by a process inwhich prepregs are stacked on one another so that a desired thickness ofthe board is obtained, and metal foils are placed, and the resultantstacked material is sandwiched between molds and inserted into a platenof a pressing machine and subjected to predetermined heating andpressing to obtain a laminated board molded, and the four sides of thelaminated board molded are cut off, followed by appearance inspection.The resin composition of the present invention can be used as asubstrate in the form of a composite material, such as glass-epoxy orTeflon-epoxy, obtained by mixing the resin composition with anothermaterial for substrate.

The resin composition of the present invention can be used as a sealer.The sealing resin is a resin material used in packaging for protecting asemiconductor chip from an external factor, such as a mechanical orthermal stress, or humidity, and the performance of a package formedfrom the resin composition of the present invention is ascribed to thethermal conductivity and weathering resistance of the cured resin.

The resin composition of the present invention can be used as a bondingagent. The bonding agent is a substance used for bonding two objectstogether, and, with respect to the material for an object to be bonded,there is no particular limitation. The bonding agent temporarily hasflowability when applied to or engaged with the surface of an object tobe bonded, and loses the flowability and is solidified after used forbonding. Examples of bonding agents include a solvent-type bondingagent, a pressure-sensitive bonding agent, a heat-sensitive bondingagent, such as an adhesive sheet, and a reactive bonding agent. When theresin composition of the present invention is used as a bonding agent,the thermal conductivity and weathering resistance of the resincomposition after used for bonding are ascribed to the thermalconductivity and weathering resistance of the cured resin.

The resin composition of the present invention can be used as a bondingagent to produce a metal base circuit board. The metal base circuitboard is produced by a process in which a bonding agent is applied to ametal plate, and a metal foil is stacked on the bonding agent in aB-stage, and the resultant stacked material is integrated bypredetermined heating and pressing.

The resin composition of the present invention can be used as aradiator. Examples of radiators include a radiating sheet, a radiatingspacer, and a radiating grease. The radiating sheet is an electricalinsulating, thermal conductive sheet used for removing heat generatedfrom a heating electronic component or electronic device, and it isproduced by adding thermal conductive filler to a silicone rubber, andmainly used in the form of being fitted to a radiating fin or a metalplate. The radiating grease is substantially similar to the radiatingsheet except that silicone oil is used instead of the silicone rubber.The radiating spacer is a silicone solid which directly transfers heatgenerated from a heating electronic component or electronic device to,e.g., a casing for electronic apparatus, and which has a thicknesscorresponding to the space between the heating electronic component orelectronic device and the casing.

EXAMPLES

The present invention will be described in more detail with reference tothe following Examples, which should not be construed as limiting thescope of the present invention.

1. Sphere-Shaped Coated Magnesium Oxide Powder

Synthesis Example 1

Magnesium oxide powder (KMAO-H, manufactured by Tateho ChemicalIndustries Co., Ltd.), which is an aggregate of single crystals having acrystallite size of 58.3×10⁻⁹ m, was ground using an impact grinder intoa particle size of 100×10⁻⁶ m or less. Fumed silica (purity: 99.9% ormore; specific surface area: 200±20 m²/g) was wet-blended with theground powder so that the amount of the fumed silica became 10 mass %,based on the mass of magnesium oxide, and they were mixed with eachother by stirring at 400 to 500 rpm for 600 s. After the stirring, theresultant mixture was subjected to filtration and dehydration to obtaina cake, and the cake was dried overnight using a dryer at 423 K. Thedried cake was milled by means of a sample mill while controlling theparticle size of the resultant powder to be similar to that of themagnesium oxide powder as a raw material, obtaining a coated magnesiumoxide powder.

Synthesis Example 2

A coated magnesium oxide powder was obtained in substantially the samemanner as in Synthesis Example 1 except that the amount of the fumedsilica was changed to 3 mass %.

Synthesis Example 3

A coated magnesium oxide powder was obtained in substantially the samemanner as in Synthesis Example 1 except that the amount of the fumedsilica was changed to 30 mass %.

Synthesis Example 4

Magnesium oxide powder (KMAO-H, manufactured by Tateho ChemicalIndustries Co., Ltd.), which is an aggregate of single crystals having acrystallite size of 58.3×10⁻⁹ m, was ground using an impact grinder intoa particle size of 100×10⁻⁶ m or less. A 4% aqueous solution of aluminumnitrate (a special grade reagent, manufactured by Kanto Chemical Co.,Inc.) was wet-blended with the ground powder so that the amount of Al₂O₃became 10 mass %, based on the mass of magnesium oxide, and they weremixed with each other by stirring at 400 to 500 rpm for 600 s. After thestirring, the resultant mixture was subjected to filtration and a cakebeing formed was satisfactorily washed with water to remove the residualaluminum nitrate, and then dehydrated to obtain a cake, and the cake wasdried overnight using a dryer at 423 K. The dried cake was milled bymeans of a sample mill while controlling the particle size of theresultant powder to be similar to that of the magnesium oxide powder asa raw material, obtaining a coated magnesium oxide powder.

Synthesis Example 5

A coated magnesium oxide powder was obtained in substantially the samemanner as in Synthesis Example 4 except that, instead of the aluminumnitrate, an aqueous solution of iron nitrate was added so that theamount of Fe₂O₃ became 15 mass %, based on the mass of magnesium oxide.

Example 1

The powder prepared in Synthesis Example 1 was fed to a high-temperatureflame formed by burning of liquefied propane gas and oxygen to effect afusing and sphere-shaping treatment, obtaining a sphere-shaped coatedmagnesium oxide powder coated with forsterite (Mg₂SiO₄).

Example 2

A fusing and sphere-shaping treatment was conducted in substantially thesame manner as in Example 1 except that the powder prepared in SynthesisExample 2 was used, obtaining a sphere-shaped coated magnesium oxidepowder coated with forsterite (Mg₂SiO₄).

Example 3

A fusing and sphere-shaping treatment was conducted in substantially thesame manner as in Example 1 except that the powder prepared in SynthesisExample 3 was used, obtaining a sphere-shaped coated magnesium oxidepowder coated with forsterite (Mg₂SiO₄).

Example 4

A fusing and sphere-shaping treatment was conducted in substantially thesame manner as in Example 1 except that the powder prepared in SynthesisExample 4 was used, obtaining a sphere-shaped coated magnesium oxidepowder coated with spinel (Al₂MgO₄).

Example 5

A fusing and sphere-shaping treatment was conducted in substantially thesame manner as in Example 1 except that the powder prepared in SynthesisExample 5 was used, obtaining a sphere-shaped coated magnesium oxidepowder coated with magnesium ferrite (Fe₂MgO₄)

Comparative Example 1

The powder obtained in Synthesis Example 1 was calcined in air at 1,723K for 3,600 s, and then further milled by means of a sample mill whilecontrolling the particle size of the resultant powder to be similar tothat of the magnesium oxide powder as a raw material, obtaining a coatedmagnesium oxide powder coated with forsterite (Mg₂SiO₄)

Comparative Example 2

Magnesium oxide powder was fed to a high-temperature flame formed byburning of liquefied propane gas and oxygen to obtain a magnesium oxidepowder having an uncoated surface.

Evaluation Test

With respect to each of the magnesium oxide powder samples obtained inExamples 1 to 5 and Comparative Examples 1 and 2, an average shapefactor, a double oxide content, a BET specific surface area, an averageparticle size, and a humidity resistance were individually measured, andthe results are shown in Table 1. The methods for measurements of theindividual items are as follows.

Average shape factor: A particle image was taken using an ultra-depthshape measurement microscope “VK8550” (manufactured by KEYENCECORPORATION), and shape factors of 100 particles were measured by meansof an image analysis software “Easy32” (manufactured by LIBRARY Co.Ltd.) and an average of the values was obtained as an average shapefactor.

Double oxide content of powder surface: A content of an element in apowder sample was measured using a scanning fluorescent X-ray analyzer“ZSX-100e” (manufactured by Rigaku Corporation) to determine a doubleoxide content.

BET specific surface area: A specific surface area of a powder samplewas measured using a flow type specific surface area measurementapparatus “FlowSorb II2300” (manufactured by Shimadzu Corporation) inaccordance with a gas adsorption method.

Average particle size: A volume average particle size of a powder samplewas measured using a particle size distribution measurement apparatus“Microtrac HRA” (manufactured by NIKKISO Co., Ltd.) in accordance with alaser diffraction and scattering method.

Humidity resistance test: 5×10⁻³ kg of the sample obtained was stirredin 100×10⁻⁶ m³ of boiling water at a temperature of 373 K for 2 hours,and a mass increase rate (mass %) was measured to evaluate a humidityresistance. TABLE 1 Sphere-shaped coated magnesium oxide Double oxideBET specific Content Average Average particle surface area Mass increaseType (mass %) shape factor size (10⁻⁶ m) (10³ m²/kg) rate (mass %)Example 1 Mg₂SiO₄ 17.49 1.186 21.01 0.75 2.97 Example 2 Mg₂SiO₄ 6.561.242 20.24 0.54 4.12 Example 3 Mg₂SiO₄ 48.52 1.154 21.77 0.83 1.37Example 4 Al₂MgO₄ 21.76 1.194 20.45 0.78 2.77 Example 5 Fe₂MgO₄ 21.001.190 20.21 0.31 1.08 Comparative Mg₂SiO₄ 18.45 1.262 20.45 0.48 3.19Example 1 Comparative — — 1.289 21.09 0.81 7.34 Example 22. Resin Composition

Example 6

Epoxysilane in an amount of 1.0 mass % was added to the sample powderprepared in Example 1, and they were mixed with each other by stirringfor 600 s to effect a surface treatment for the powder, followed bydrying at 423 K for 7,200 s. 560 parts by weight of the resultantsample, 63 parts by weight of an ortho-cresol novolak epoxy resin, 34parts by weight of a novolak phenolic resin, 1 part by weight oftriphenylphosphine, and 2 parts by weight of carnauba wax were mixedtogether and milled using a mill for 600 s. Then, the resultant mixturewas kneaded using a twin roll at 373 K for 300 s, and then the kneadedmixture was further ground into 10 mesh or less to prepare pellets of φ38 mm×t 15 mm. The pellets were subjected to transfer molding under 7MPa at 448 K for 180 s, and a spiral flow was measured by the methodshown below.

Separately, the pellets were subjected to transfer molding under 7 MPaat 448 K for 180 s, and then subjected to post cure at 453 K for 18×10³s to obtain a molded article of φ 50 mm×t 3 mm.

Example 7

A spiral flow was measured and a molded article was obtained insubstantially the same manner as in Example 6 except that thesphere-shaped coated magnesium oxide powder prepared in Example 2 wasused.

Example 8

A spiral flow was measured and a molded article was obtained insubstantially the same manner as in Example 6 except that thesphere-shaped coated magnesium oxide powder prepared in Example 3 wasused.

Example 9

A spiral flow was measured and a molded article was obtained insubstantially the same manner as in Example 6 except that thesphere-shaped coated magnesium oxide powder prepared in Example 4 wasused.

Example 10

A spiral flow was measured and a molded article was obtained insubstantially the same manner as in Example 6 except that thesphere-shaped coated magnesium oxide powder prepared in Example 5 wasused.

Comparative Example 3

A spiral flow was measured and a molded article was obtained insubstantially the same manner as in Example 6 except that the sampleprepared in Comparative Example 1 was used.

Comparative Example 4

A spiral flow was measured and a molded article was obtained insubstantially the same manner as in Example 6 except that alumina powderwas used instead of the magnesium oxide powder.

Example 11

Vinyltrimethoxysilane in an amount of 1.0 mass % was added to the samplepowder prepared in Example 1, and they were mixed with each other bystirring for 600 s to effect a surface treatment for the powder,followed by drying at 423 K for 7,200 s. 451 parts by weight of theresultant sample and 100 parts by weight of a two-pack RTV siliconerubber were kneaded together using a twin roll for 300 s. Then, 5 partsby weight of a platinum catalyst was added to the kneaded mixture, andthey were kneaded using a twin roll for 600 s to prepare a compound, anda viscosity was measured under the conditions shown below. The compoundwas subjected to press molding under 5 MPa at 393 K for 600 s to obtaina molded article of φ 50 mm×t 3 mm.

Comparative Example 5

A viscosity was measured and a molded article was obtained insubstantially the same manner as in Example 11 except that the samplepowder prepared in Comparative Example 1 was used.

Comparative Example 6

A viscosity was measured and a molded article was obtained insubstantially the same manner as in Example 11 except that aluminapowder was used instead of the magnesium oxide powder.

Evaluation Test

With respect to each of the resin compositions obtained in Examples 6 to11 and Comparative Examples 3 to 6, a spiral flow or viscosity (methodwas appropriately selected depending on the state of the resin at roomtemperature) was measured, and, with respect to the molded article ofeach resin composition, a thermal conductivity, a humidity resistance,and appearance after the humidity resistance test were individuallymeasured, and the results are shown in Table 2. The evaluation methodsfor the individual items are as follows.

Spiral flow: Measured in accordance with EMMI-1-66.

Viscosity: A viscosity was measured using a rheometer “VAR-50”(manufactured by REOLOGICA Instruments AB) at a shear rate of 1 s⁻¹.

Thermal conductivity: A thermal conductivity of a molded article wasmeasured using a thermal constant measurement apparatus “TC-3000”(manufactured by SHINKU-RIKO, Inc.) in accordance with a laser flashmethod.

Humidity resistance test: A molded article was stored in athermo-hygrostat at a temperature of 358 K at a humidity of 85% for 7days, and a moisture absorption rate was measured. Further, theappearance of the resultant molded article was visually inspected. TABLE2 Evaluation tests of molded article Flowability Appearance of resinMoisture after composition Thermal absorption humidity Double SpiralViscosity conductivity rate resistance oxide Resin flow (m) (Pa · s)(W/mK) (mass %) test Example 6 Mg₂SiO₄ Epoxy 0.507 — 3.11 0.18 No changeresin Example 7 Mg₂SiO₄ Epoxy 0.448 — 3.25 0.20 No change resin Example8 Mg₂SiO₄ Epoxy 0.653 — 3.03 0.11 No change resin Example 9 Al₂MgO₄Epoxy 0.535 — 3.12 0.15 No change resin Example 10 Fe₂MgO₄ Epoxy 0.492 —3.15 0.14 No change resin Comparative Mg₂SiO₄ Epoxy 0.343 — 3.18 0.17 Nochange Example 3 resin Comparative —⁽*⁾ Epoxy 0.502 — 2.78 0.15 Nochange Example 4 resin Example 11 Mg₂SiO₄ Silicone —  471 2.20 0.20 Nochange rubber Comparative Mg₂SiO₄ Silicone — 3280 2.14 0.20 No changeExample 5 rubber Comparative —⁽*⁾ Silicone — 1130 1.70 0.16 No changeExample 6 rubber⁽*⁾Uncoated Al₂O₃ powder was used instead of coated MgO powder.

As is apparent from the results shown above, the sphere-shaped coatedmagnesium oxide powders meeting the average shape factor defined in thepresent invention (Table 1, Examples 1 to 5) have excellent humidityresistance. Further, it has been confirmed that the resin compositionscomprising the above powder (Table 2, Examples 6 to 11) have excellentflowability, and that the molded articles formed from the resincompositions have high thermal conductivity and excellent humidityresistance.

On the other hand, the powder in Comparative Example 1 has excellenthumidity resistance, but it has an average shape factor as large as 1.25or more. Both the epoxy resin containing this powder (Table 2,Comparative Example 3) and the silicone rubber containing this powder(Table 2, Comparative Example 5) had low flowability.

The powder in Comparative Example 2 was not coated with a double oxideand hence, as seen in Table 1, had a very low humidity resistance.

The resin compositions comprising conventional alumina powder instead ofthe magnesium oxide powder (Table 2, Comparative Examples 4 and 6) hadexcellent flowability and excellent humidity resistance, but they hadpoor thermal conductivity.

INDUSTRIAL APPLICABILITY

As described above in detail, the sphere-shaped coated magnesium oxidepowder of the present invention has excellent humidity resistance, andfurther has excellent filling property when being used as filler, andhence can fill a resin with high density, and therefore the powder isadvantageously used as thermal conductive filler.

The resin composition comprising the sphere-shaped coated magnesiumoxide powder has excellent flowability, and the molded article formedfrom the resin composition has high heat radiating property and highhumidity resistance, and therefore it is remarkably advantageously usedas a sealer, a spacer, a bonding agent, or an adhesive sheet for use ina variety of electronic devices, or a constituent member, such as aresin circuit board, a metal base circuit board, a metal-clad laminate,or a metal-clad laminate having an inner circuit, and thus it is highlyvaluable from a commercial point of view.

1. A sphere-shaped coated magnesium oxide powder having a surface coatedwith a double oxide and having an average shape factor of 1.25 or less.2. The sphere-shaped coated magnesium oxide powder according to claim 1,wherein said double oxide has a melting point of 2,773 K or lower. 3.The sphere-shaped coated magnesium oxide powder according to claim 2,wherein said double oxide comprises at least one element selected fromthe group consisting of aluminum, iron, silicon, and titanium, andmagnesium.
 4. The sphere-shaped coated magnesium oxide powder accordingto claim 1, which contains said double oxide in an amount of 5 to 50mass %.
 5. The sphere-shaped coated magnesium oxide powder according toclaim 1, which has an average particle size of 5×10⁻⁶ to 500×10⁻⁶ m anda BET specific surface area of 5×10³ m²/kg or less.
 6. A method forproducing a sphere-shaped coated magnesium oxide powder, comprisingallowing a compound of an element forming a double oxide to be presenton the surface of magnesium oxide powder, and then fusing the resultantmagnesium oxide powder at a high temperature so that the surface of themagnesium oxide powder is coated with the double oxide and the magnesiumoxide powder is shaped into sphere.
 7. The method according to claim 6,wherein the compound of the element forming a double oxide together withmagnesium is at least one compound selected from the group consisting ofan aluminum compound, an iron compound, a silicon compound, and atitanium compound.
 8. The method according to claim 6, wherein themagnesium oxide powder to be coated has a crystallite size of 50×10⁻⁹ mor more.
 9. The method according to claim 6 wherein the temperature is aflame temperature of 2,073 K or higher.
 10. A resin compositioncomprising the sphere-shaped coated magnesium oxide powder according toclaim
 1. 11. The resin composition according to claim 10, wherein theresin in the resin composition is an epoxy resin.
 12. The resincomposition according to claim 10, wherein the resin in the resincomposition is a silicone rubber.
 13. An electronic device using theresin composition according to claim 10.